I'm in the process of building an eBay store. How likely is it that this will turn into a full-time business for me?
I think the likelihood of this becoming a full-time gig has more to with you and your business model than it does with eBay. If you've developed something unique and coupled that with great pricing and service, then eBay's a great distribution and marketing system to reach people you wouldn't otherwise reach.
Still, you have to remember that eBay's not magic. The real magic is your brain, hard work, and whether or not your business model is a good one. If it is, then eBay becomes a cool methodology through which your idea gets to market.
Regardless of how you're executing on the internet, you still have to be executing the business properly. You still have to use basic marketing and have good margins, prices and service. The big thing is you still have to love on your customers a lot.
Plenty of people think the internet is an easy path to fame and fortune, and they forget about the basics of building and handling an operation well on a day-to-day basis. Those are the ones who don't last long in the business world, whether it's online or brick-and-mortar!